On the effect of distortion and dispersion in fringe signal of the FG5 absolute gravimeters

Abstract

The knowledge of absolute gravity acceleration at the level of 1 × 10 −9 is needed in geosciences (e.g. for monitoring crustal deformations and mass transports) and in metrology for watt balance experiments related to the new SI definition of the unit of kilogram. The gravity reference, which results from the international comparisons held with the participation of numerous absolute gravimeters, is significantly affected by qualities of instruments prevailing in the comparisons (i.e. at present, FG5 gravimeters). Therefore, it is necessary to thoroughly investigate all instrumental (particularly systematic) errors. This paper deals with systematic errors of the FG5#215 coming from the distorted fringe signal and from the electronic dispersion at several electronic components including cables. In order to investigate these effects, we developed a new experimental system for acquiring and analysing the data parallel to the FG5 built-in system. The new system based on the analogue-to-digital converter with digital waveform processing using the FFT swept band pass filter is developed and tested on the FG5#215 gravimeter equipped with a new fast analogue output. The system is characterized by a low timing jitter, digital handling of the distorted swept signal with determination of zero-crossings for the fundamental frequency sweep and also for its harmonics and can be used for any gravimeter based on the laser interferometry. Comparison of the original FG5 system and the experimental systems is provided on g-values, residuals and additional measurements/models. Moreover, advanced approach for the solution of the free-fall motion is presented, which allows to take into account a non-linear gravity change with height.

Evaluation of comparisons of absolute gravimeters using correlated quantities: reprocessing and analyses of recent comparisons

“…An upgrade of a gravimeter from FG5 to FG5X is indicated by a gap in connecting lines and by different markers. In case of FG5-215, the gravimeter was not upgraded to FG5X but equipped with a new measurement system described in Křen et al (2016) establishment of a long-term stable gravity reference frame, compliant with the requirements in geodesy and metrology.…”

“…Table 1), possible common systematic errors might affect the reference. Therefore, it is necessary to investigate systematic errors of the individual technologies, as in case of FG5/X gravimeters mainly the diffraction effect (Van Westrum and Niebauer 2003;Křen and Pálinkáš 2018) and the distortion effect (Křen et al 2016) and in case of cold-atom gravimeters (e.g. Pereira and Bonvalot 2016) the wavefront aberration (Schkolnik et al 2015).…”

Experiment with simultaneous measurements at two optical wavelengths in the FG5X absolute gravimeter

“…This system, mounted with an additional photodetector in the complementary output of the interferometer, could work in parallel with the original detection system. The FFT swept filtering method allows a software-based separation of the interferometric signals from two lasers with sufficiently different wavelengths [9]. The principle of such a wavelength separation is shown in figure 2.…”

“…Thus, it is important to analyse potential systematic errors of these types of gravimeters and carefully elaborate the associated uncertainty budget. Several error sources of FG5/X gravimeters have been detected in the past as the self-attraction [7] and recently the diffraction [8], distortion [9] and verticality effects [10].…”

The results of CCM.G-K2.2017 key comparison

“…Vertical gravity gradients (VGGs) were calculated from gravity differences measured with Scintrex CG-6#S032, ZLS Burris#B095 and ZLS Burris#B101 gravimeters at 4 different vertical levels above all the sites, as shown in Figure 3. This same procedure was adopted from the ICAG-2009 in BIPM [3]. Based on the results obtained from three relative gravimeters, we approximated gravity changes with height at all sites by second order…”